This application claims the priority of Germany Application No. 19637410.3-34, filed Sep. 13, 1996, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a device for resistance spot welding of sheets or shapes of a material which is a good conductor of electricity and heat (especially aluminum, magnesium or copper), with a displaceable foil section provided between an electrode cap of the welding electrode and the surface of the shape or the sheet (especially aluminum sheet).
The term "shape" is intended to include extrusion-molded shapes that can be welded by spot welding electrodes, as well as filigree-cast thin-walled parts made of such materials.
When conventional aluminum sheets are welded, an oxide layer can be located on the surface of the sheets, and can also be removed for example by pickling or grinding. The term "conventional aluminum sheet" is intended to include sheets with conventional coatings, for example a titanium-zirconium coating. The basic material of the sheets can be in the 5 series or the 6 series, in other words it can be AlMg3 to AlMg5 (international conventional designation: 5182) or it can be AlMg0.4 Si 1,2 (international conventional designation: 6016).
In addition to aluminum sheets, which are known to have relatively high electrical conductivity as well as relatively high thermal conductivity, the electrode of the present invention is especially well suited for welding sheets or filigree shapes made of other materials which are likewise marked by relatively high conductivity. In addition to aluminum-coated steel sheets, examples of other such materials are copper (especially silver-coated copper) and magnesium. In addition, a casting skin can occur on the surfaces of shapes that are cast primarily from these materials; such skin, however, is removable by grinding as well.
German patent document DE 44 16 504 A1 describes a welding arrangement in which the foil section (preferably a component of a foil strip) protects the electrode during welding of aluminum sheets against pickup of aluminum.
As is known, during welding, molten material can be seen spraying out at the contact area between the welding electrode and the aluminum sheet, which causes mixed crystal formation between the copper part of the welding electrodes and the aluminum. This in turn results in the deposition of electrode material on the sheet surface, and also of sheet material on the electrode. The latter phenomenon is referred to as pickup.
When welding aluminum sheets with conventional electrode caps made of copper alloys, such pickup is known to damage the electrode surface, shortening its lifetime and weakening the spot weld to the point that it is no longer sufficient. It also can cause the electrodes to stick together and even damaging the surface of the sheet.
By providing a foil section made of a material that is a good conductor of both electricity and heat between the electrode cap and the surface of the sheet, (and replacing it after one or more spot welds), such pickup on the electrode cap can be prevented, thereby considerably increasing its lifetime. As indicated in the abovementioned document, the foil section can be replaced especially simply if it is part of a foil strip unwound from a coil, stretched across the electrode, and guided for example to a second coil, so that it can be advanced periodically as the foil section abutting the electrode cap wears away.
However, in tests of welding of ordinary aluminum sheet using obvious and conventional materials for the foil strip, adhesion was observed either between the foil section and the electrode cap or between the foil section and the aluminum sheet. These adhesion effects are disadvantageous however since they either negatively influence the quality of the spot weld, or in the worst case actually prevent the foil strip from being advanced relative to the electrode cap.
In addition, such adhesion effects have also been observed in the welding of sheets, or especially precision-cast filigree shapes, composed of the other materials mentioned above.
The object of the present invention, therefore, is to provide a welding arrangement with which not only conventional aluminum sheet in particular, but also sheet or spot-weldable shapes made of other materials that are particularly good conductors, can be resistance-spot-welded, with interposition of a foil section that can be advanced relative to the electrode cap when signs of wear appear.
This object is achieved according to the invention by providing a foil section which consist of a material with a high nickel content, and either coating the electrode cap (which consists primarily of a copper alloy) with silver or a silver metal oxide or providing it with an insert made of silver or a silver metal oxide that comes in contact with the foil section. In particular, the electrode cap can be hard-silvered.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.